1. Field of the Invention
This invention relates to testing arc fault detectors including those used in circuit breakers. Specifically, it is directed to an arc fault detector test circuit which employs a low frequency relaxation oscillator for simple verification of the operability of the detector.
2. Background Information
Arc faults can occur in electrical systems, for instance, between adjacent bared conductors, between exposed ends of broken conductors, a faulty connection, and in other situations where conducting elements are in close proximity. Arc faults typically have high resistance so the arc current is below the instantaneous or magnetic trip thresholds of conventional circuit breakers. Also, arc faults create an average RMS current value which is below the thermal threshold for such circuit breakers. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads which draw similar currents, and would therefor, cause nuisance trips.
Thus, various types of arc fault detectors have been proposed. Some of these detectors respond to the step change in current randomly occurring each time an arc is struck. One such detector is disclosed in U.S. Pat. No. 5,459,630. This detector counts the number of step changes in current which occur in a given time interval. U.S. Pat. No. 5,459,630 also discloses a test circuit for testing the arc fault detector. The particular circuit disclosed utilizes one of the coils of the dormant oscillator ground fault circuit also provided in the circuit breaker. A capacitor forms with the sensing coil an oscillator when a test button is closed. Another test circuit suggested by U.S. Patent No. 5,459,630 includes an impedance, such as a resistor, which is connected across the line and neutral conductors to produce a pulse each time a test switch is closed. This test circuit requires repeated manual actuation of the test switch to generate test pulses which are then counted by the arc fault detector.
An improved arc fault detector of the type which responds to the step changes in current produced by the striking of the arc, is described in U.S. Pat. No. 5,691,869. An arc fault signal which can be used to trip a circuit breaker is generated when a time attenuated accumulation of the pulses generated each time an arc is struck reaches a selected magnitude. Thus, fewer arcs which produce large step increases in current are required to trip the circuit breaker than smaller amplitude arcs.
While test circuits such as those described in U.S. Pat. No. 5,459,630 could be applied to the arc fault detectors of U.S. Pat. No. 5,691,869, there is a need for improved arc fault test circuits.
More particularly, there is a need for an improved test circuit for an arc fault detector which automatically generates step increases mimicing the striking of an arc, and therefore, does not require repetitive actuation to generate a series of "simulated" arcs.
There is a more particular need for such an arc fault detector test circuit which provides more information about the operation of the test circuit.
In particular, there is a need for such a test circuit which provides an indication of the number of simulated arc strikes which generated the trip.
There is a further need for such a test circuit which alternatively, or additionally, allows an operator to manually time the time to trip using the test circuit.